1. Field of the Invention
The present invention relates to a power converter and more specifically relates to a primary-side controlled power converter.
2. Description of Related Art
Various modes of control for power converters have been widely used to regulate an output voltage and an output current. Two common modes for such regulation are voltage-mode and current-mode.
FIG. 1 shows a traditional power converter. An error amplifier 12 and an error amplifier 13 are utilized to produce a feedback signal from the output current and the output voltage. The feedback signal will be provided to a PWM controller 11 in the primary-side circuit via an optical-coupler 14. The PWM controller 11 generates a PWM signal to switch a transformer 51 by switching a transistor 70. A switching current flowing through the transistor 70 is converted to a detection voltage signal via a resistor 20. The PWM controller 11 compares the feedback signal with the detection voltage signal to produce a PWM signal. The PWM signal will regulate the output voltage and/or the output current. However, the optical-coupler and other secondary-side control circuitry add significantly to the size and device count of the power converter. This is a main drawback of this power converter.
FIG. 2 shows the schematic circuit of a primary-side controlled power converter. The primary-side controlled power converter includes a PWM controller 12. A reflected-voltage from an auxiliary winding NA of a transformer 50 is supplied to the PWM controller 12 via a resistor 61. This is used to provide the feedback signal for output voltage control. Since the reflected-voltage produced by the auxiliary winding NA is strongly correlated with an output voltage provided by a secondary winding NS, the PWM controller 12 can regulate the output voltage from the primary-side of the transformer 50.
To regulate the output current from the primary-side of the transformer 50, it is necessary to control the power delivered from the primary-side to the secondary-side of the transformer 50. The relationship between an output power PO and a switching current IIN of the power converter can be expressed as:
                              P          O                =                                            V              O                        ×                          I              O                                =                                    η              ×                              P                IN                                      =                          η              ×                              I                IN                            ×                              V                IN                                                                        (        1        )                                          I          IN                =                  [                                    (                                                                    V                    IN                                                        L                    P                                                  ×                                                      T                    ON                                                        2                    ⁢                    T                                                              )                        +                          (                                                I                  A                                ×                                                      T                    ON                                    T                                            )                                ]                                    (        2        )            
Where VO is the output voltage; IO is the output current; η is the efficiency; IA is the reflected load current; LP is the primary magnetized inductance; T is the switching period; and TON is the on-time of the switching signal.
In order to control the output voltage VO, the on-time TON of the switching signal is adjusted in response to the feedback signal. When the output current IO increases, a switching current of the transistor will also increase. The switching current of the transistor 70 will be converted to a switching-voltage. When the switching-voltage exceeds a maximum threshold voltage VLIMIT, the on-time TON of the switching signal will be restricted to regulate the output current IO The maximum input power PIN—MAX can be expressed as:
                              P          IN_MAX                =                  [                                    1                              2                ×                T                                      ×                          L              P                        ×                                          (                                                      V                    LIMIT                                                        R                    S                                                  )                            2                                ]                                    (        3        )            
The equations (1) and (3) can then be rewritten as:
                              I          O                =                                            P              O                                      V              O                                =                                                    η                ×                                  P                  IN_MAX                                                            V                O                                      =                          {                                                η                  ×                                      [                                                                  1                                                  2                          ×                          T                                                                    ×                                              L                        P                                            ×                                                                        (                                                                                    V                              LIMIT                                                                                      R                              S                                                                                )                                                2                                                              ]                                                                    V                  O                                            }                                                          (        4        )            
Because the maximum input power PIN—MAX is limited, the output voltage VO will decrease whenever the output current IO increases. The output voltage deviation and the output current variation (the slope of the V/I curve) determine the constant current output performance. FIG. 3 provides an illustration of this.
Referring to equation (4), when the output voltage VO decreases, a constant output current IO can be achieved by increasing the switching period T and/or reducing the voltage signal VLIMIT. However, any of several factors can skew the accuracy of this method. Deviation in the primary magnetized inductance LP and a drifting switching frequency (1/T) can cause the maximum input power limit PIN—MAX and the output current IO to fluctuate.
Furthermore, the propagation delay time TD affects the precision of the output current IO. Referring to FIG. 2, a resistor 20 is used to convert the switching current IIN to a switching-current voltage VIDET. As shown in FIG. 4, a voltage VIA can be expressed as:VIA=IA×RsWhere IA is the reflected load current; RS is the resistance of the resistor 20.
The switching-current voltage VIDET exceeds the threshold voltage VLIMIT at the time TONX. However, VPWM is not turned off until after a propagation delay time TD. During this propagation delay time TD, the switching current IIN continues to increase. This will generate an extra input current IIN-ex.
The amplitude of the current IIN-ex can be calculated according to equation (5). Referring to equation (6), the extra input current IIN-ex will cause the maximum input power PIN-MAX and the output current IO to increase in response to an increasing input voltage VIN.
                              I                      IN            -                          θ              ⁢                                                          ⁢              x                                      =                  {                                    (                                                                    V                    IN                                                        L                    P                                                  ×                                                      T                    D                                                        2                    ⁢                    T                                                              )                        +                                          [                                                      (                                                                                            V                          IN                                                                          L                          P                                                                    ×                                              T                        ONX                                                              )                                    +                                      I                    A                                                  ]                            ×                                                T                  D                                T                                              }                                    (        5        )                                          P          IN_MAX                =                  [                                    1                              2                ×                T                                      ×                          L              P                        ×                                          (                                                                            V                      LIMIT                                                              R                      S                                                        +                                      I                                          IN                      -                      ex                                                                      )                            2                                ]                                    (        6        )            
According to foregoing description, we can find that the primary-side control of the prior art is not accurate and is not fit for the mass production of the power converter.
The object of the present invention is to provide a primary-side controlled power converter that can precisely regulate the output voltage and generate a constant output current.